Vacuum ultraviolet transmitting direct deposit vitrified silicon oxyfluoride lithography glass photomask blanks

ABSTRACT

High purity direct deposit vitrified silicon oxyfluoride glass suitable for use as a photomask substrates for photolithography applications in the VUV wavelength region below 190 nm is disclosed. The inventive direct deposit vitrified silicon oxyfluoride glass is transmissive at wavelengths around 157 nm, making it particularly useful as a photomask substrate at the 157 nm wavelength region. The inventive photomask substrate is a dry direct deposit vitrified silicon oxyfluoride glass which exhibits very high transmittance in the vacuum ultraviolet (VUV) wavelength region while maintaining the excellent thermal and physical properties generally associated with high purity fused silica. In addition to containing fluorine and having little or no OH content, the inventive direct deposit vitrified silicon oxyfluoride glass suitable for use as a photomask substrate at 157 nm is also characterized by having less than 1×10 17  molecules/cm 3  of molecular hydrogen and low chlorine levels.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to, and the benefit of, U.S.Provisional Patent Application No. 60/200,405 filed Apr. 8, 2000entitled “Water-Free Fused Silica And Method Therefor,” and U.S.Provisional Patent Application No. 60/258,132 filed Dec. 22, 2000entitled “Substantially Dry, Silica-Containing Soot, Fused Silica AndOptical Fiber Soot Preforms, Apparatus, Methods And Burners ForManufacturing Same And Method Therefor,” and U.S. Provisional PatentApplication No. 60/271,136 filed Feb. 24, 2001, entitled VacuumUltraviolet Transmitting Silicon Oxyfluoride Lithography Glass, and U.S.Provisional Patent Application No. 60/271,135, filed Feb. 24, 2001,entitled Oxygen Doping of Silicon Oxyfluoride Glass, and PCT ApplicationWO 01/17919, Sep. 8, 2000, entitled Pure Fused Silica, Furnace AndMethod, the disclosures of which are hereby incorporated by referenceherein.

FIELD OF THE INVENTION

The present invention relates generally to lithography, and particularlyto optical photolithography glass for use in optical photolithographysystems utilizing vacuum ultraviolet light (VUV) wavelengths below 193nm, preferably below 175 nm, preferably below 164 nm, such as VUVprojection lithography systems utilizing wavelengths in the 157 nmregion.

The invention relates to VUV transmitting glass that is transmissive atwavelengths below 193 nm, in particular, a photomask silicon oxyfluorideglass suitable for use in the Vacuum Ultraviolet (VUV) 157 nm wavelengthregion.

BACKGROUND OF THE INVENTION

Refractive optics requires materials having high transmittance. Forsemi-conductor applications where smaller and smaller features aredesired at the 248 and 193 nm wavelengths, high purity fused silica hasbeen show to exhibit the required minimum transmittance of 99%/cm orbetter.

Projection optical photolithography systems that utilize the vacuumultraviolet wavelengths of light below 193 nm provide benefits in termsof achieving smaller feature dimensions. Such systems that utilizevacuum ultraviolet wavelengths in the 157 nm wavelength region have thepotential of improving integrated circuits with smaller feature sizes.Current optical lithography systems used by the semiconductor industryin the manufacture of integrated circuits have progressed towardsshorter wavelengths of light, such as the popular 248 nm and 193 nmwavelengths, but the commercial use and adoption of vacuum ultravioletwavelengths below 193 nm, such as 157 nm has been hindered by thetransmission nature of such vacuum ultraviolet wavelengths in the 157 nmregion through optical materials. Such slow progression by thesemiconductor industry of the use of VUV light below 175 nm such as 157nm light has been also due to the lack of economically manufacturablephotomask blanks from optically transmissive materials. For the benefitof vacuum ultraviolet photolithography in the 157 nm region such as theemission spectrum VUV window of a F₂ excimer laser to be utilized in themanufacturing of integrated circuits there is a need for mask blanksthat have beneficial optical properties including good transmissionbelow 164 nm and at 157 nm and that can be manufactured economically.

The present invention overcomes problems in the prior art and provides aeconomical high quality improved photomask blanks and VUV transmittinglithography glass that can be used to improve the manufacturing ofintegrated circuits with vacuum ultraviolet wavelengths.

Use of high purity fused silica as optical elements in photolithographystems from the fact that high purity fused silica is transparent over awide range of wavelengths, spanning from the infrared to deepultraviolet regions. Furthermore, high purity fused silica exhibitsexcellent chemical durability and dimensional stability. It has beensuggested in EP 0 636 586 A1 that in order to be suitable for use as aphotomask substrate for certain photolithography applications at 248 and193 nm wavelengths, high purity fused silica made by the direct flamemethod must contain high molecular hydrogen in the range of 10¹⁷ to 10¹⁹molecules/cm³. Similarly, JP 1-201664 discloses that synthetic quartzglass for use as photomask material whose optical properties have beenchanged due to sputtering, plasma etching or excimer irradiation, can berestored to its original condition by heat treating the glass in ahydrogen atmosphere. Specifically, this document describes the effect onsynthetic quartz of exposure to 248 and 193 nm wavelengths. The effectof exposure to 248 and 193 nm wavelengths on fused silica is alsodescribed in “Densification of Fused Silica under 193 nm excitation,” byBorrelli et al, in J. Opt. Soc. Am. B/Vol. 14, No. 7, pp. 1606-1615(July 1997); and by Allan et al., in “193-mn excimer-laser-induceddensification of fused silica,” Optics Letters, Vol. 21, No. 24, pp.1960-1962 (Dec. 5, 1996).

EP 0 901 989 A1 discloses a manufacturing method for making silica glasssubstantially free of chlorine. In a direct deposit concurrentvitrifying process silicon tetrafluoride is flame hydrolyzed to providea silica glass in which fluorine is controlled within the range 100 ppmto 450 ppm and OH group density in the range from 600 ppm to 1300 ppm.

U.S. Pat. No. 5,326,729 discloses quartz glass having excimer laserresistance produced by subjecting the glass to dehydration treatment ina temperature range lower than the transparent vitrification temperatureof the glass followed by transparent vitrification and molding to adesired shape, followed by a doping treatment in a hydrogen atmosphere.

U.S. Pat. No. 5,474,589 discloses a UV light permeable fluorine-dopedsynthetic quartz glass with decreased defects.

Applicants, previously have disclosed several effective methods forimproving the optical properties of high purity fused silica when usedas an optical lens in photolithography at both the 248 and 193 runwavelength regions. See for example, U.S. Pat. Nos. 5,616,159; 5,668,067and 5,735,921 all incorporated herein by reference.

Accordingly, it is an object of the present invention to disclose VUVtransmitting dry direct deposit vitrified silicon oxyfluoride glassesfor use at VUV wavelengths below 193 nm, preferably in the F₂ ExcimerLaser 157 nm region, methods of making such glass, and methods of makingdry direct deposit vitrified lithography glass articles.

SUMMARY OF THE INVENTION

In the present invention we disclose VUV transmitting dry direct depositvitrified silicon oxyfluoride lithography glass suitable for use asoptical elements, for use as a lens or preferably for use as a photomasksubstrate at VUV wavelengths below 193 nm. In particular, the inventivedirect deposit vitrified silicon oxyfluoride glass production exhibitsbenifits tailored for optical lithography articles and applications inthe photolithography VUV wavelength region around the 157 nm Excimerlaser wavelengths and below 193 nm.

The object of the invention is achieved by use of a dry low hydroxyradical fluorine-doped SiO₂ fused direct deposit vitrified syntheticsilicon oxyfluoride glass which exhibits high transmittance in thevacuum ultraviolet (VUV) wavelength region while exhibiting excellentthermal and physical properties. By “dry” we mean having an OH contentbelow 50 ppm by weight, preferably dehydrated below 10 ppm OH by weight,and most preferably below 1 ppm by weight.

In another aspect, the object of the invention is further achieved byensuring that the dry direct deposit vitrified silicon oxyfluoride glassis essentially free of chlorine.

In yet another aspect, the object of the invention is achieved byensuring a low molecular hydrogen content in the dry direct depositvitrified glass. By this we mean that the molecular hydrogen (H₂)content is below 1×10¹⁷ molecules/cm³. In a preferred embodiment of theinvention, the VUV transmitting dry direct deposit vitrified siliconoxyfluoride glass has a fluorine content in the range of 0.1 to 0.4weight percent which inhibits laser exposure induced absorption andprovides laser exposure durability with minimal transmission loss at157.6 nm after prolonged exposure. The invention includes a below 193 nmVUV transmitting glass photomask substrate for photolithography atwavelengths of about 157 nm with the glass being a high purity drydirect deposit vitrified silicon oxyfluoride glass with an OH contentbelow 50 ppm by weight, hydrogen content below 1×10¹⁷ molecules/cm³ anda fluorine content in the 0.1 to 0.4 weight percent range.

The invention includes a process of making VUV transmitting glasssilicon oxyfluoride glass that includes providing a hydrogen-free fuelcarbon monoxide combustion burner; providing a heat containing directdeposit furnace; providing a supply of carbon monoxide and a supply ofoxygen to said carbon monoxide combustion burner to form a carbonmonoxide combustion reaction flame, providing a direct glass depositionsurface proximate said flame, supplying a Si-glass precursor feedstockand a F-glass precursor feedstock to said carbon monoxide combustionburner wherein said Si-glass precursor feedstock and said F-glassprecursor feedstock is reacted in said flame into a silicon oxyfluorideglass soot directed at said glass deposition surface, and said soot isconcurrently directly deposited and vitrified into a dry direct depositvitrified silicon oxyfluoride glass body.

The invention includes a dry direct deposit vitrified siliconoxyfluoride glass having essentially no OH groups, less than 5×10¹⁶molecules/cm³ of molecular hydrogen, and a fluorine content in the rangeof 0.1 to 0.4 weight %. The invention includes a dry direct depositvitrified silicon oxyfluoride lithography glass having an OH contentless than 5 ppm by weight, a Cl content less than 5 ppm by weight, a H2content less than 1×10¹⁷ molecules/cm³, and a fluorine content of 0.1 to0.4 weight % with a 157 nm internal transmission of at least 85%/cm. Theinvention includes a VUV pattern printing method with the steps ofproviding a below 164 nm radiation source for producing VUV photons,providing a dry direct deposit vitrified silicon oxyfluoride glasshaving less than 5 ppm by weight OH, less than 5 ppm by weight Cl, a<0.5weight percent fluorine content, and 157 nm and 165 nm measuredtransmission of at least 75%/5 mm. The pattern printing method includestransmitting the VUV photons through the dry direct deposit vitrifiedsilicon oxyfluoride glass, forming a pattern with the VUV photons andprojecting the pattern onto a VUV radiation sensitive printing pattern.The invention includes a dry direct deposit vitrified VUV transmittingsilicon oxyfluoride glass having a OH content less than 5 ppm by weight,a fluorine content of at least 0.1 weight %, the glass consistingessentially of Si, O, and F with an internal transmission in thewavelength range of 157 nm to 175 nm of at least 85%/cm and a 165 nmabsorption less than 0.4 (absorption units/5 mm) after exposure to a 157nm laser for 41.5 million pulses at 2 mJ/cm²-pulse.

The invention includes a below 193 nm VUV transmitting glass photomasksubstrate for photolithography at wavelengths of about 157 nm, saidglass photomask substrate comprising a dry high purity direct depositvitrified silicon oxyfluoride glass with an OH content below 20 ppm byweight, a Cl content below 0.1% by weight, and a fluorine content in therange of 0.01 to 7 weight percent.

The invention includes a method of making a below 193 nm VUVtransmitting optical lithography glass for transmitting wavelengths ofabout 157 nm, said method comprising providing a hydrogen-free fuelcarbon monoxide combustion burner; providing a supply of carbon monoxideand a supply of oxygen to said carbon monoxide combustion burner to forma carbon monoxide combustion reaction flame, providing a direct glassdeposition surface proximate said flame, supplying a Si-glass precursorfeedstock and a said F-glass precursor feedstock to said carbon monoxidecombustion burner wherein said Si-glass precursor feedstock and saidF-glass precursor feedstock is reacted in said flame into a siliconoxyfluoride glass soot directed at said glass deposition surface, andsaid soot is concurrently directly deposited and vitrified into asilicon oxyfluoride glass body.

The invention includes a method of making a homogeneous glass opticallithography element, said method comprising providing a hydrogen-freefuel carbon monoxide combustion burner; providing a supply of carbonmonoxide and a supply of oxygen to said carbon monoxide combustionburner to form a carbon monoxide combustion reaction flame, providing adirect glass deposition surface proximate said flame, supplying aSi-glass precursor feedstock and a dopant R-glass precursor feedstock tosaid carbon monoxide combustion burner wherein said Si-glass precursorfeedstock and said dopant R-glass precursor feedstock is reacted in saidflame into a dry R doped silica glass soot directed at said glassdeposition surface, and said soot is concurrently directly deposited andvitrified into a dry homogeneous R doped silica glass body, and formingsaid directly deposited vitrified glass body into a homogeneous glassoptical lithography element. In preferred embodiments of the method ofmaking dry R doped silica glass, the glass dopant R is chosen from theglass dopant group consisting of F, Ti, Ge, B, P, and Al.

The invention includes a method of making a homogeneous glass opticallithography element, said method comprising providing a hydrogen-freefuel carbon monoxide combustion burner; providing a supply of carbonmonoxide and a supply of oxygen to said carbon monoxide combustionburner to form a carbon monoxide combustion reaction flame, providing adirect glass deposition surface proximate said flame, supplying aSi-glass precursor feedstock to said carbon monoxide combustion burnerwherein said Si-glass precursor feedstock is reacted in said flame intoa dry high purity silica glass soot directed at said glass depositionsurface, and said soot is concurrently directly deposited and vitrifiedinto a dry homogeneous high purity silica glass body, and forming saiddirectly deposited vitrified glass body into a homogeneous glass opticallithography element.

The method of making below 193 nm VUV transmitting silicon oxyfluorideglass includes providing a hydrogen-free fuel combustion burner, withthe preferred hydrogen-free fuel combustion burner being a carbonmonoxide combustion burner. The method includes providing a supply ofhydrogen-free carbon monoxide fuel and a supply of oxygen to thecombustion burner to form a carbon monoxide combustion reaction flamewhich is contained within a heat containing direct deposit furnace. Adirect glass deposition surface is provided in said furnace proximateand preferably below said carbon monoxide burner and flame. Alternativehydrogen-free fuel combustion fuels include carbon suboxide and carbonylsulfide. a supply of carbon monoxide and oxygen to the combustion burnermaintains the carbon monoxide flame to which is supplied a Si-glassprecursor feedstock and a F-glass precursor feedstock. Preferably theSi-glass precursor feedstock is hydrogen free, such as silicontetrachloride and silicon tetraisocyanate [Si(NCO)₄].

The process and apparatus in accordance with one embodiment of theinvention manufactures substantially water-free silica glass. Theprocess and apparatus to make such water-free fused silica glass does soby eliminating the possibility of water ever forming in the combustionatmosphere. This is achieved in a first embodiment thereof by utilizinga substantially hydrogen-free fuel, such as carbon monoxide (CO), carbonsuboxide (C₃O₂), carbonyl sulfide (COS), and the like. Use of suchsubstantially H-free fuels minimizes water formation in the combustionreaction. According to a preferred embodiment, it is desired to use asubstantially hydrogen-free raw material as a glass precursor for silicaalso. Most preferably, a combination of substantially hydrogen-free rawmaterial and substantially hydrogen-free fuel is utilized. Typicalexamples of substantially H-free glass precursors include siliconcarbide (SiC), silicon monoxide (SiO), silicon nitride (Si₃N₄), silicontetrabromide (SiBr₄), silicon tetrachloride (SiCl₄), silicon tetraiodide(SiI₄) and silica (SiO₂). Si(NCO)₄ may also be utilized.

In accordance with the invention, when carbon monoxide, for example, isused as the fuel and combined with oxygen, the only by-product is carbondioxide. This by-product is easily disposed of and, advantageously, nowater is formed from the process reaction. This reaction is illustratedby the following equation.

CO+½O₂→CO₂

It was recognized that the available heat from carbon monoxide is aboutone-fourth the heat available from natural gas (methane). Therefore,four times the fuel would be required to produce the same amount ofheat. However, only one-half mole of combustion supporting oxygen isrequired to combust one mole of CO. Thus, the total volume of oxygenrequired is the same for either fuel to produce the same amount of heat.The following equation shows the required carbon monoxide fuel needed tomatch the available heat of combusting one mole of methane (CH₄) used inone prior art process.

4CO+2O₂→4CO₂

The equation below shows the by-products and combustion supportingoxygen needed for combustion of one mole of methane in the prior art.

CH₄+2O₂→2H₂O+CO₂

Thus, from the foregoing, it should be recognized that the production ofsubstantially water-free silica glass is obtainable.

A method for producing a vitrified glass article is provided by theinvention. The inventive method comprising several steps. First, heat isgenerated from a combustion burner having a flame produced by igniting asubstantially hydrogen-free fuel. According to the invention, the flameis the only source of heat. Next, a glass precursor is flowed into theflame to produce silica-containing soot. Finally, the silica-containingsoot is deposited onto a substrate and substantially simultaneouslyconverted (by the heat of the flame) to form the vitrified glass articleby the heat of the flame. In a preferred embodiment, soot is depositedonto a silica-containing glass member, such as a fused silica puck.According to this method, the vitrified glass article contains very lowamounts of water. The step of depositing preferably takes place within afurnace chamber that may include a purge gas, such as nitrogen providedthereto. This method is adapted for producing homogeneous glass.According to the invention, a hydrogen-free fuel carbon monoxidecombustion burner is utilized. The burner comprises a fume passageadapted to supply, at a first flow rate, a glass precursor, and a fuelpassage surrounding the fume passage, the fuel passage adapted to supplya substantially hydrogen-free fuel at a flow rate at least 20 times thefirst flow rate. The burner may also include an inner shield passagebetween the fuel passage and the fume passage adapted to supply at leastoxygen. The burner may further comprise an outer shield passagesurrounding the fuel passage for introduction of additional gasses.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary of the invention, andare intended to provide an overview or framework for understanding thenature and character of the invention as it is claimed. The accompanyingdrawings are included to provide a further understanding of theinvention, and are incorporated in and constitute a part of thisspecification. The drawings illustrate various embodiments of theinvention, and together with the description serve to explain theprincipals and operation of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

We have shown that high transmission at below 190 nm, in particularbelow 175 nm, and most preferably at the F₂ Excimer Laser wavelengthoutput centered around 157 nm can be provided in SiO₂ containing siliconoxyfluoride glasses by minimizing the OH or water content of the glass.Specifically, we have demonstrated that low OH low chlorine siliconoxyfluoride glass exhibits high transmissivity.

The transmission properties of glass in general are dependent on glasscomposition. In pure silica, it has been demonstrated that even tracelevels (ppm or less) of metal contaminants can cause significantreductions in transmission in the ultraviolet region. We havedemonstrated that besides metal impurities, the most important variablesfor controlling the VUV transmission edge of silicon oxyfluoride glassinclude its water or OH content, as well as its chlorine content.Specifically, we have found that the lower the OH content, the betterthe transmission, while the higher the chlorine content, the lower thetransmission in the VUV 157 nm region. In addition, we have found thatthe amount of molecular hydrogen in the glass has to be minimized. Mostpreferably the SiO₂ containing silicon oxyfluoride glass has at least0.5 wt. % Fluorine. In an alternatively preferred embodiment the siliconoxyfluoride glass has a fluorine content in the range of 0.1 to 0.4weight percent.

In a preferred embodiment measurements of the infrared transmission ofglass at 2.7 microns is used to quantify the OH content of glass.

An exemplary embodiment of a dry high purity direct deposit vitrifiedsilicon oxyfluoride glass lithography photomask substrate of the presentinvention is shown in FIGS. 1-2 and is designated generally throughoutby reference numeral 20.

As shown in FIG. 2, in accordance with the invention, the presentinvention for a photolithography method for producing lithographypatterns includes the step of providing an illumination subsystem forproducing and directing a <300 nm UV radiation λ. Preferably λ<200 nm,and most preferably λ<193 nm. The method includes providing a masksubsystem with a mask stage and a transmitting photolithography mask 22which includes a dry high purity direct deposit vitrified siliconoxyfluoride glass wafer 20 with photolithography pattern depictions 24.The method includes providing a projection optics subsystem andproviding a λ radiation sensitive print subsystem, which includes aradiation sensitive print media 26. As shown in FIG. 2, the methodfurther includes aligning the illumination subsystem, the masksubsystem, the projection optics subsystem, and the radiation sensitiveprint subsystem, and illuminating mask 22 with the radiation λ withradiation λ traveling through glass 20 such that the photolithographypattern depictions IC of glass wafer mask 22 are projected onto media26. FIG. 3 shows mask 22 with photolithography pattern depictions IC ondirect deposit vitrified silicon oxyfluoride glass substrate 20.Photolithography IC patterns such as shown in FIG. 4 are formed frommask 22 with radiation λ transmitted through glass 20, transmittedthrough the projection optics and with the IC pattern projected ontointegrated circuit wafer media 26 as shown in FIG. 5. The inventivephotolithography method includes transmitting photolithography patterndepictions in the form of UV photons with <193 nm wavelengths throughthe glass wherein attenuation of the radiation λ photolithography lightis inhibited. In a preferred embodiment, the method includes providingan F₂ excimer laser 28 which produces UV radiation λ and λ includes alaser emission wavelength of 157 nm as shown in FIG. 6.

In a preferred embodiment, below 193 nm VUV transmitting glass photomasksubstrate 20 for photolithography at wavelengths of about 157 nm is adry high purity direct deposit vitrified silicon oxyfluoride glass withan OH content below 20 ppm by weight, a Cl content below 0.1% by weight,and a fluorine content in the range of 0.01 to 7 weight percent.

Preferably dry high purity direct deposit vitrified silicon oxyfluorideglass 20 has a fluorine content in the range of 0.01 to 2 weightpercent, more preferably a fluorine content in the range of 0.01 to 0.5weight percent, and most preferably a fluorine content in the range 0.1to 0.4 weight percent. Preferably dry high purity direct depositvitrified silicon oxyfluoride glass 20 has a Cl content≦0.08% by weight,more preferably a Cl content≦0.06% by weight, more preferably a Clcontent≦0.04% by weight, more preferably a Cl content≦0.03% by weight,more preferably a Cl content≦0.02% by weight. In preferred embodimentsdry high purity direct deposit vitrified silicon oxyfluoride glass 20has a Cl content below 5 ppm, more preferably a Cl content below 1 ppm,and most preferably the direct deposit vitrified silicon oxyfluorideglass is Cl-free with no detectable chlorine.

Preferably dry high purity direct deposit vitrified silicon oxyfluorideglass 20 has an OH content below 10 ppm by weight, and more preferablyan OH content below 1 ppm by weight.

Preferably the dry high purity direct deposit vitrified siliconoxyfluoride glass has a hydrogen content below 1×10¹⁷ molecules/cm³.Preferably dry high purity direct deposit vitrified silicon oxyfluorideglass has a Fe content no greater than 0.00004% by weight. Preferablythe dry high purity direct deposit vitrified silicon oxyfluoride glasshas a Zr content no greater than 0.00004% by weight.

In a preferred embodiment dry high purity direct deposit vitrifiedsilicon oxyfluoride glass 20 contains a plurality of intersticial O₂molecules, preferably with an O₂ concentration of at least 10¹⁵ O₂mole/cc, preferably ≧10¹⁶ O₂ moles/cc, preferably ≦10²⁰ O₂ moles/cc, andmost preferably in range of about 10¹⁶ to 10¹⁰ O₂ moles/cc.

In a preferred embodiment of the invention glass substrate 20 is anon-piece-annealed glass wafer in that the glass piece 20 has not beenannealed in its glass wafer substrate physical form state. In apreferred embodiment the glass is annealed in a preform physical statethat is distinct from and larger than the glass wafer glass substratepiece, preferably with the glass being annealed as a glass preform disk32 (FIG. 7), with the physical size of the glass preform that isannealed being considerably larger than that of glass wafer 20 (muchlarger volume and much larger longest dimension; at least twice,preferably at least thrice, more preferably at least quadruple).

As shown in FIG. 1 a lithography photomask blank glass substrate 20 hasa longest dimension length L. As shown in FIG. 7 direct depositvitrified silicon oxyfluoride glass preform disk 32 has a preform diskdiameter D and a preform disk height H with D>H. As shown in FIG. 7, thepreferred direct deposit vitrified silicon oxyfluoride glass preformdisk 32 has a height H and a diameter D, with diameter D lying in planexy defined by preform disk x-axis and preform disk y-axis, and thex-axis and the y-axis oriented normal to preform disk height H. Diskheight H is in alignment with the preform disk z-axis. As shown in sideview process flow FIGS. 8a-c, the method includes identifying aninclusion free region 34 in preform disk 32 with preform disk inclusionfree region 34 including glass free of inclusions having diametersgreater than 1 μm. As shown in FIGS. 8a and 8 b preferably the inventionincludes maintaining the preform disk x-axis, y-axis, and z-axisorientation while removing inclusion free region 34 from preform disk 32to provide a photomask blank preform 36 having a photomask blank preformx-axis in alignment with the preform disk x-axis, a photomask blankpreform y-axis in alignment with the preform disk y-axis, and aphotomask blank preform disk z-axis. As shown in FIGS. 8b, 8 c and 1,the method includes forming photomask blank preform 36 into alithography photomask blank 20 having a longest dimension length L. In apreferred method of the invention, lithography photomask blank 20 has athickness T, a lithography photomask blank x-axis, a lithographyphotomask blank y-axis, and a lithography photomask blank z-axis withthe lithography photomask blank x-axis and the lithography photomaskblank y-axis in alignment with the photomask blank preform x-axis andthe photomask blank preform y-axis. The longest dimension length L oflithography photomask blank 20 lies in plane xy defined by photomaskblank x-axis and y-axis and the lithography photomask blank thickness Tis aligned with the lithography photomask blank z-axis and normal to thephotomask blank x-axis and y-axis and the thickness T is less than L.Preferably T is much smaller than L, more preferably 10T<L.

With the inventive method the direct deposit vitrified glass preform 32has a flat geometry, preferably a flat disk shape versus a tallcolumnary shape where the height in the z axis orientation is largerthan the base dimension in the xy plane. Such a preform with a flatgeometry shape versus a tall geometry shape provide a preferred glassbody with uniform optical characteristics that provide improvedlithography performance. In a preferred embodiment providing directdeposit vitrified glass preform disk 32 includes providing preform disk32 with a longest dimension diameter D that is greater than or equal totwice the height H (D≧2H), more preferably with D≧3H, and mostpreferably D≧4H.

The method of making below 193 nm VUV transmitting glass photomasksubstrate 20 includes making a direct deposit vitrified siliconoxyfluoride glass body 32. FIG. 9 and FIG. 10 show methods of making andproviding direct deposit vitrified glass preform disk 32. In a preferredembodiment the high purity Si glass precursor feedstock is delivered invapor form through delivery conduits 38 to carbon monoxide conversionsite burner carbon monoxide combustion flames 40 at heat containingdirect deposit conversion site furnace 42 which converts the Si and Fcontaining feedstocks into dry silicon oxyfluoride glass soot 44 whichis directed at and deposited on direct glass deposition surface 45,which preferably is contained in revolving horizontally orientedcollection cup 46 and soot 44 is concurrently directly depositedvitrified consolidated into high purity dry silicon oxyfluoride glassbody 48. Preferably heat containing direct deposit conversion sitefurnace 42 is constructed of refractory bodies made from aluminumdioxide which have been halogen cleansed of contamination contents.Furnace 42 including cup 46 is preferably comprised of halogen treatedcleansed aluminum dioxide refractory blocks which are free ofcontaminants and preferably which are obtained by removing thecontaminants prior to forming glass, such as carbo-chlorination with ahalogen containing cleaning/contaminant removing atmosphere. Preferredlow contaminant bubbled alumina refractories are disclosed in PCTApplication WO 01/17919, Sep. 8, 2000, entitled Pure Fused Silica,Furnace And Method by Kotacska et al., which is incorporated byreference. Rotating cup 46 is oriented horizontally (parallel to xyplane, normal to z axis), preferably in addition to rotation cup 46, cup46 is moved in the xy plane using an x-y oscillation table with xyoscillation movement patterns. As shown in FIG. 10 it is preferred tominimize and inhibit changes in gas flow and the environment withinfurnace 42 so that a consistent glass body 48 is produced. A preferreddirect deposit furnace is disclosed in U.S. Pat. No. 5,951,730, Sep. 14,1999, by Paul Schermerhorn, which is incorporated by reference. Thetemperature within heat containing furnace 42 is maintained at a hightemperature to ensure consolidation of soot 44 into glass body 48 as itis deposited, preferably the operating temperature of furnace 42 andglass body 48 is at least 1500° C., more preferably at least 1600° C.,and most preferably at least 1650° C.

Along with such high temperatures which allow the glass body to flow ina preferred embodiment cup 46 is constructed with sloping side walls asshown in FIG. 10 which are not as steep as those in FIG. 9 and promotebeneficial movement and flow of the glass. Preferred collection cupcontainment vessels are disclosed in U.S. Pat. No. 5,698,484, Dec. 16,1997 by John Maxon, which is incorporated by reference. Preferablyhorizontally oriented collection cup 46 has a collection cup height CCHand a collection cup diameter CCD, with CCH>H and CCD>D. Providing glasspreform disk 32 includes discarding the periphery of glass body 48,particularly the periphery of glass body 48 in contact with cup 46, sothat photomask blanks originate from the non-outer periphery parts ofglass body 48. As shown in FIGS. 9-10, glass body 48 is preferablycontained within direct deposit collection cup 46. In addition tocontaining the physical form of the flowable glass, collection cup 46protects the glass body from changes and influences of the surroundingenvironment and most preferably inhibits heat loss from the glass bodyin that cup 46 is formed from a refractory insulating material, andparticularly reduces heat loss from the sides and bottom of glass body48 with heat being generated above glass body 48 by the carbon monoxideflames of carbon monoxide combustion burners 40, and from auxiliary heatsources such as carbon monoxide combustion heat source burners that arealso located in furnace conversion site 42 above glass body 48 and cup46. In a preferred embodiment, the invention includes continuouslydepositing soot 44 in cup 46 while concurrently consolidating the sootin order to direct deposit vitrify a fused glass body while maintainingthe temperature of the building up body at a temperature of at least1500° C. Built up glass body 48 is preferably maintained at suchtemperature needed for consolidation vitrification (consolidationvitrification temperature) with the temperature of the entire glass bodybeing substantially homogeneous and even throughout. Preferably suchtemperatures are maintained by minimizing heat loss from the glass body48 throughout its production. The flat disk geometry such as with glasspreform disk having D≧2H, preferably helps to minimize heat loss fromthe surfaces of glass body 48. The formation of such flat disk shapedbodies, preferably within an insulating containing cup, minimize heatloss through the side of the body opposite the heat source (bottom ofbody 48 is opposite of heat sources on top of furnace 42) and from thesides of body 48. Such flat disk shapes beneficially inhibit heat lossthrough the bottom and sides compared to geometry's based on long tallcolumns. As shown in FIGS. 9-10, it is preferred that soot 44 isdirected at deposition surface 45 and travels down into cup 46 along adownward deposition path from the carbon monoxide combustion flames ofcarbon monoxide burners 40, and revolving cup 46 is rotated in a planeof rotation substantially perpendicular to the downward deposition pathof soot 44. The plane of rotation of cup 46 is parallel with the planedefined by the preform disk x-axis and the preform disk y-axis. Inaddition to revolving motion in the rotation plane parallel to the xyplane, cup 46 is translated in an oscillation motion in such a xyparallel plane preferably utilizing oscillation rotation patterns asdisclosed in U.S. Pat. No. 5,696,038, Dec. 9, 1997, Boule OscillationPatterns In Methods of Producing Fused Silica Glass by John Maxon, whichis incorporated by reference.

In a preferred embodiment glass preform disk 32 is annealed inside acarbon monoxide direct deposit furnace 42 after the formation of glassbody 48 is completed and production and deposition of soot 44 is ended.In a preferred method of making lithography photomask blank 20, theglass is not annealed after removal from preform disk 32 in thatphotomask blank preform 36 and individual blanks 20 are not annealed. Inthe preferred embodiment any birefringence present in the glass isreduced in the large physical size of preform disk 32 and body 48 andnot during processing after the preform disk.

Forming photomask blank preform 36 into lithography photomask blanks 20preferably includes cutting a plurality of photomask blanks from preform36 and polishing the cut photomask blanks. The invention furtherincludes forming a lithographic pattern on photomask blank 20 andtransmitting below 193 nanometer wavelength radiation through saidphotomask blank.

In a preferred practice of the invention preform disk 32 has a diametergreater than 20 inches (50 cm) such as D about 3 to 5 feet (0.91 to 1.5meters) and a height H of about 6 to 10 inches (15 to 25 cm) with themask blanks 20 made therefrom having a longest dimension L<12 inches (30cm) such as blank dimensions of about 10 inches×10 inches (25 cm×25 cm),about 9 inches×9 inches (22.8 cm×22.8 cm) and about 6 inches×6 inches(15 cm×15 cm), with thickness T of about ¼ inch (0.63 cm). Numerous maskblanks 20 can be cut from the larger preform disk 32, with the largepreform disk size providing improved lithography performance,particularly compared to photomask substrates formed from small preformcolumns that have near net preform dimensions close to the dimensions ofthe photomask substrates. FIG. 11 illustrates a process flow of apreferred method of making mask blanks 20. Preform disk 32 is providedsuch as shown in FIGS. 9-10, with a carbon monoxide conversion flamedirect deposit vitrification lay down process and employingoscillation/rotation of cup 46. The location of photomask blank preforms36 is layed out on preform disk 32, preferably with the center of disk32 avoided. As shown in FIG. 11 the location layout of photomask blankpreforms 36 is preferably staggered to avoid any human eye visuallydetectable inclusions. A non-staggered checker board aligned column-rowformations can be used without inspecting for detectable inclusions inpreform disk 32 with inclusion containing glass discarded later in theprocess. After the location layout of photomask blank preforms 36 aredetermined the photomask blank preform blocks 36 are cut out of preformdisk 32. A representative photomask blank preform block 36 has a squarebase of about 6½ inch×6½ inch (16.5 cm×16.5 cm) and a height of about5-6 inches (12 cm to 15 cm). The cut out blocks 36 are polished on threesides to allow inspection and mapping of the glass interior. Theinterior is inspected using the three polished sides by transmittinglight through the two opposing polished sides in order to identify, markand map any inclusions over 1 μm in size which may be in the interiorvolume of the glass. An optical measurement system is preferably used toscan an inspection laser light beam (HeNe scan beam) 101 through thevolume of block 36 to identify inclusions, with the inclusion observedthrough the third polished side, so that the location of the inclusioncan be mapped and marked for subsequent removal. A method and systemsuch as disclosed in U.S. Ser. No. 09/458,561, filed Dec. 9, 1999,entitled Automated System For Measurement Of An Optical Property,Attorney Docket No. Priestley-1, by Richard Priestley, which isincorporated by reference, can be used. Block 36 is then sliced intoblank slabs, with the slabs sliced so that identified inclusions areremoved. Inclusions are cut around with the cut blank slabs having athickness of about 0.4-0.5 inches (1-1.3 cm). The cut blank slabs arethen pre-finished with chemical-mechanical finishing, flatness lap, flatplate flatness polishing and edge polishing to provide a pre-finishedmask blank. The pre-finished mask blank is measured for optics. In analternative embodiment the optics of the glass can be measured in theblock form as done for inclusion inspection. A method and system such asdisclosed in U.S. Ser. No. 09/458,561, filed Dec. 9, 1999, entitledAutomated System For Measurement Of An Optical Property, Attorney DocketNo. Priestley-1, by Richard Priestley, which is incorporated herein, canbe used for optic measurements. The pre-finished mask blank is thengiven a final finish to provide finished glass substrate photomask blank20. The final finish preferably includes chemical mechanical polishingto a super polish below 5 angstroms finish and flatness, cleaned andpackaged for incorporation into a mask 22.

As shown in FIG. 12, preferably to obtain sufficient heat from the dryCO flame, substantially hydrogen-free CO fuel 126 and glass feedstockprecursor 124 are preferably supplied at a predetermined flow ratio. Inparticular, to generate sufficient heat, the flow of fuel to the flow ofglass precursor 124 should be greater than 20:1. This is accomplished,by proper sizing of the various passages within the combustion burner125. One preferable combustion burner is illustrated in FIG. 13. The COcombustion burner adapted for combusting substantially hydrogen-free gasshall be referred to herein as a “dry combustion burner.” The drycombustion CO burner 125 includes a center fume tube 68, formed as aslender tube, and is adapted to supply the gaseous glass feedstockprecursor. Preferably surrounding the fume tube 68 is an inner shieldpassage 74 that is adapted to carry oxygen. Oxygen, a combustionsupporting gas, is supplied in a ratio of fuel to combustion supportinggas of about 2:1. Surrounding the fume tube 68 and inner shield 74 isthe fuel passage 70 adapted to carry the large volumes of substantiallyhydrogen-free CO fuel. Although not shown exactly to scale, it isapparent that the cross-sectional area of the fuel passage 70 is muchlarger than of the fume tube 68. Because, for example, carbon monoxidecontains less heat when ignited, higher flows are required as comparedto methane. This is designed such that the glass precursor 124 may besupplied at a first flow rate to a center fume passage 68 of thecombustion burner 125 and that the substantially hydrogen-free fuel 126may be supplied at a flow rate at least 20 times the first flow ratethereby enabling generation of sufficient heat to oxidize the precursor.The burner 125 may include multiple input ports for supplying thesubstantially hydrogen-free fuel 126 and the combustion supportingoxygen 21 thereby providing more uniform flow distribution in theannular shaped passages.

Fluorine may be incorporated into the soot in another embodiment of theinvention. There are several ways that this may be accomplished inaccordance with the invention. First, the fluorine may be included inthe precursor, such as when a chlorofluorosilane is used for theprecursor 124. In this scenario, the precursor 124 is supplied as a gasto the fume tube 68 and oxidized by the CO flame thereby producingfluorine doped soot. Alternatively, some fuel or oxygen may be suppliedwith the substantially hydrogen-free fuel.

A second way of introducing fluorine is by flowing fluorine or afluorine-containing compound such as of F₂, CF₄, C₂F₆, SF₆, NF₃, SiF₄ orcombinations thereof in gaseous feedstock form into a shield includedwithin the combustion burner. FIG. 14 illustrates a burner 125 a thatmay be utilized to produce silicon oxyfluoride soot. Fluorine or thefluorine-containing feedstock compound is supplied in gaseous form toouter shield passage 72 surrounding the fuel passage 70. A water coolingjacket may be utilized surrounding the fuel passage. A preferredembodiment of the CO combustion burner has a center tube 68 adapted toprovide a substantially hydrogen-free glass precursor into a flameregion, the center tube located along a central axis of the burner 125,125 a (FIGS. 13-14); an inner shield unit 74 adapted to provide oxygeninto the flame, the inner shield unit radially displaced from thecentral axis of the burner, a fuel unit 70 radially displaced from thecentral axis of the burner and adapted to provide asubstantially-hydrogen free fuel; and an outer shield unit 72 adapted toprovide a fluorine containing gas enshrouding the flame, the outershield region radially displaced from the central axis of the burner andpositioned outside the inner shield unit and the fuel unit, the burnerbeing adapted for producing substantially water-free, fluorine dopedsilica.

In accordance with illustrated embodiment of FIG. 12, the substantiallyhydrogen free fuel may be utilized for making a glass article, such as adisc of glass 86. Silicon-containing gas feedstock molecules are reactedin CO flame 128 to form soot particles. These particles 130 aredeposited on the hot direct deposition surface of a body 132 where theyconsolidate into a very viscous fluid (deposited and virtuallysimultaneously vitrified) which is later cooled to the solid state.

According to another embodiment of the invention, a method for producinga dry vitrified glass article is provided. The method preferablycomprises the steps of generating heat from a combustion burner 125having a flame 128 that is produced by igniting a substantiallyhydrogen-free fuel 126, hydrogen-free CO fuel flame 128 being the sourceof heat, flowing a glass precursor 124 into the flame 128 to producesilica containing soot 130, and depositing the silica containing sootonto a deposition surface substrate 132 and substantially simultaneouslyvitrifying consolidating the soot to form the vitrified glass article86. In accordance with a preferred embodiment, the soot is depositedonto a substrate 132 that is itself a silica-containing glass member,and most preferably a dry glass disc of the invention. By utilizing thesubstantially hydrogen free fuel in accordance with the invention, thevitrified glass article 86 contains water (OH) in amount less than aboutseveral ppm. In the illustrated embodiment, the step of depositing takesplace within a heat containing furnace chamber 89. Preferably, a purgegas, such as nitrogen, is provided into the chamber such that asubstantially water free environment is provided. Generally, it isdesirable to provide a pressurized atmosphere in the chamber 89 greaterthan an atmospheric pressure outside of the chamber.

In a preferred embodiment, providing a supply of carbon monoxide to saidcarbon monoxide combustion burner to form a carbon monoxide combustionreaction flame includes providing a high purity carbon monoxide supplygas and passing said high purity carbon monoxide supply gas through aninline CO purifier filter 300 upstream of said carbon monoxidecombustion burner. The inline CO purifier filter 300 upstream of saidcarbon monoxide combustion burner removes moisture, iron, andcontaminating metals from the high purity carbon monoxide beingdelivered to the burner to ensure a high purity flame and resulting highpurity glass formed therefrom. A CO purifier filter such as Waferpure®Micro/Mini-XL/Megaline Gas Purifiers brand CO purifier type fromMillipore Corporation is a suitable CO purifier filter. Preferrably thehigh purity carbon monoxide supply gas is at least 99.3% pure CO.

The method of making a below 193 nm VUV transmitting glass photomasksubstrate blank for photolithography at wavelengths of about 157 nm,includes providing a hydrogen-free fuel carbon monoxide combustionburner; providing a heat containing direct deposit furnace; providing asupply of carbon monoxide and a supply of oxygen to said carbon monoxidecombustion burner to form a carbon monoxide combustion reaction flame,providing a direct glass deposition surface proximate said CO flame,supplying a Si-glass precursor feedstock and a F-glass precursorfeedstock to said carbon monoxide combustion burner wherein saidSi-glass precursor feedstock and said F-glass precursor feedstock isreacted in said CO flame into a silicon oxyfluoride glass soot directedat said glass deposition surface, and said soot is concurrently directlydeposited and vitrified into a silicon oxyfluoride glass body. Themethod includes forming said directly deposited vitrified siliconoxyfluoride glass body into a photomask blank. Preferably supplying aSi-glass precursor feedstock includes supplying a chlorine-free Si-glassprecursor feedstock wherein said directly deposited vitrified siliconoxyfluoride glass is a chlorine-free (preferably Cl<1 ppm) siliconoxyfluoride glass and said glass body is formed into a chlorine-freesilicon oxyfluoride glass photomask blank. Preferably supplying aSi-glass precursor feedstock includes supplying a hydrogen-free Si-glassprecursor feedstock wherein said directly deposited vitrified siliconoxyfluoride glass is a dry silicon oxyfluoride glass with an OH weightconcentration<10 ppm OH and said glass body is formed into a dry siliconoxyfluoride glass photomask blank with an OH weight concentration<10 ppmOH. Most preferably supplying a Si-glass precursor feedstock includessupplying a chlorine-free hydrogen-free Si-glass precursor feedstockwherein said directly deposited vitrified silicon oxyfluoride glass is adry chlorine-free silicon oxyfluoride glass with an OH weightconcentration<10 ppm OH and said glass body is formed into a drychlorine-free silicon oxyfluoride glass photomask blank with an OHweight concentration<10 ppm OH. In a prefered embodiment thechlorine-free hydrogen-free Si-glass precursor feedstock is silicontetraisocyanate. Supplying a F-glass precursor feedstock preferablyincludes supplying a predetermined F-glass precursor feedstock flowwherein said directly deposited vitrified silicon oxyfluoride glass hasa fluorine weight concentration in the range from 0.01 to 7 wt. % F,more preferably from 0.01 to 2 wt. % F, more preferably from 0.01 to 0.5wt. % F. Preferably providing a supply of oxygen including providing apredetermined O₂ supply flow wherein said directly deposited vitrifiedsilicon oxyfluoride glass contains O₂, preferably with a molecular O₂concentration of at least 10¹⁵ O₂ mole/cc.

Providing a heat containing direct deposit furnace preferably includesproviding a heat containing direct deposit furnace comprised of halogentreated cleansed aluminum dioxide refractory blocks and assembling saidhalogen treated cleansed aluminum dioxide refractory blocks to form thefurnace, most preferably with the refractory blocks consistingessentially of aluminum dioxide. The heat containing direct depositfurnace is constructed of alumina refractory materials which has beenpre-exposed to a reactive, halogen-containing gas to react with, andthereby cleanse the refractory of contaminating metals. Preferably thefurnace crown containing and adjacent the CO burners consistsessentially of aluminum dioxide, with the crown covering theconsolidating glass mass.

Preferably the furnace cup consisting essentially of aluminum dioxidecontains the consolidating glass mass. Preferably the aluminum dioxiderefractory blocks are exposed to a reactive halogen-containing gascleansing treatment. Preferably the aluminum dioxide refractory blocksare porous blocks having a porousity in the range of 25-70%, preferablywith a bulk density<3.9 grams/cm³, and more preferably with the aluminumdioxide refractory bulk density≧1.2 grams/cm³.

The invention includes a method of making a below 193 nm VUVtransmitting optical lithography glass for transmitting wavelengths ofabout 157 nm. The method includes providing a hydrogen-free fuel carbonmonoxide combustion burner; providing a supply of carbon monoxide and asupply of oxygen to said carbon monoxide combustion burner to form acarbon monoxide combustion reaction flame, providing a direct glassdeposition surface proximate said flame, supplying a Si-glass precursorfeedstock and a said F-glass precursor feedstock to said carbon monoxidecombustion burner wherein said Si-glass precursor feedstock and saidF-glass precursor feedstock is reacted in said flame into a siliconoxyfluoride glass soot directed at said glass deposition surface, andsaid soot is concurrently directly deposited and vitrified into asilicon oxyfluoride glass body. Preferably supplying a Si-glassprecursor feedstock includes supplying a chlorine-free Si-glassprecursor feedstock wherein the directly deposited vitrified siliconoxyfluoride glass is a chlorine-free (Cl<1 ppm) silicon oxyfluorideglass. Preferably supplying a Si-glass precursor feedstock includessupplying a hydrogen-free Si-glass precursor feedstock wherein saiddirectly deposited vitrified silicon oxyfluoride glass is a dry siliconoxyfluoride glass with an OH weight concentration<10 ppm OH. Preferablysupplying a Si-glass precursor feedstock includes supplying achlorine-free hydrogen-free Si-glass precursor feedstock wherein saiddirectly deposited vitrified silicon oxyfluoride glass is a drychlorine-free silicon oxyfluoride glass with an OH weightconcentration<10 ppm, most preferably wherein said chlorine-freehydrogen-free Si-glass precursor feedstock is silicon tetraisocyanate.Supplying a F-glass precursor feedstock preferably includes supplying apredetermined F-glass precursor feedstock flow wherein said directlydeposited vitrified silicon oxyfluoride glass has a fluorine weightconcentration in the range from 0.01 t 7 wt. % F, more preferably 0.01to 2 wt. % F, more preferably 0.01 to 0.5 wt. % F. Providing a supply ofoxygen preferably includes providing a predetermined O₂ supply flowwherein said directly deposited vitrified silicon oxyfluoride glass hasa molecular O₂ concentration of at least 10¹⁵ O₂ mole/cc.

The invention includes a method of making a homogeneous glass opticallithography element, with the method including providing a hydrogen-freefuel carbon monoxide combustion burner; providing a supply of carbonmonoxide and a supply of oxygen to said carbon monoxide combustionburner to form a carbon monoxide combustion reaction flame, providing adirect glass deposition surface proximate said flame, supplying aSi-glass precursor feedstock and a dopant R-glass precursor feedstock tosaid carbon monoxide combustion burner wherein said Si-glass precursorfeedstock and said dopant R-glass precursor feedstock is reacted in saidflame into a dry R doped silica glass soot directed at said glassdeposition surface, and said soot is concurrently directly deposited andvitrified into a dry homogeneous R doped silica glass body, and formingsaid directly deposited vitrified glass body into a homogeneous glassoptical lithography element. Preferably dopant R-glass precursorfeedstock is chosen from the R dopant group consisting of F, Ti, Ge, B,P, and Al. Supplying a Si-glass precursor feedstock preferably includessupplying a chlorine-free Si-glass precursor feedstock wherein saiddirectly deposited vitrified glass is a chlorine-free (Cl<1 ppm) glassand said glass body is formed into a chlorine-free homogenous glassoptical element. Supplying a Si-glass precursor feedstock preferablyincludes supplying a hydrogen-free Si-glass precursor feedstock whereinsaid directly deposited vitrified silicon oxyfluoride glass is a dryglass with an OH weight concentration<10 ppm OH and said glass body isformed into a dry homogenous glass optical element with an OH weightconcentration<10 ppm OH. Most preferably supplying a Si-glass precursorfeedstock includes supplying a chlorine-free hydrogen-free Si-glassprecursor feedstock wherein said directly deposited vitrified glass is adry chlorine-free silicon oxyfluoride glass with an OH weightconcentration<10 ppm OH and said glass body is formed into a drychlorine-free homogeneous glass element with an OH weightconcentration<10 ppm OH, more preferably wherein said chlorine-freehydrogen-free Si-glass precursor feedstock is silicon tetraisocyanate.

The invention includes a method of making a homogeneous glass opticallithography element, with method comprising providing a hydrogen-freefuel carbon monoxide combustion burner; providing a supply of carbonmonoxide and a supply of oxygen to said carbon monoxide combustionburner to form a carbon monoxide combustion reaction flame, providing adirect glass deposition surface proximate said flame, supplying aSi-glass precursor feedstock to said carbon monoxide combustion burnerwherein said Si-glass precursor feedstock is reacted in said flame intoa dry silica glass soot directed at said glass deposition surface, andsaid soot is concurrently directly deposited and vitrified into a dryhomogeneous silica glass body, and forming said directly depositedvitrified glass body into a homogeneous glass optical lithographyelement. Preferably supplying a Si-glass precursor feedstock includessupplying a chlorine-free Si-glass precursor feedstock wherein saiddirectly deposited vitrified glass is a chlorine-free (Cl<1 ppm) glassand said glass body is formed into a chlorine-free homogenous glassoptical element. Preferably supplying a Si-glass precursor feedstockincludes supplying a hydrogen-free Si-glass precursor feedstock whereinsaid directly deposited vitrified glass is a dry glass with an OH weightconcentration<10 ppm OH and said glass body is formed into a dryhomogenous glass optical element with an OH weight concentration<10 ppmOH, preferably OH weight concentration<1 ppm. In a preferred embodimentsupplying a Si-glass precursor feedstock includes supplying achlorine-free hydrogen-free Si-glass precursor feedstock wherein saiddirectly deposited vitrified silicon glass is a dry chlorine-freesilicon-glass with an OH weight concentration<10 ppm OH and said glassbody is formed into a dry chlorine-free homogeneous glass element withan OH weight concentration<10 ppm OH, preferably OH weightconcentration<1 ppm.

Since fluorine is a desirable component of the inventive fused silica,fluorine-containing F-glass precursor feedstocks starting material suchas silicon fluoride, silicon tetrafluoride and mixtures of these, may beused in the methods of the invention. In addition to utilizing siliconfluoride starting materials, fluorine source starting material F-glassprecursor feedstocks such as CF₄, SF₆, F₂, C₃F₈ and C₂F₆ can be usedalong with silicon-containing starting material compound Si-glassprecursor feedstocks reacting to produce fluorine doped silica fromsilicon fluoride silica soots.

In addition to being preferably free of OH groups, being fluorine-dopedand essentially free of chlorine, we have found that the inventive fusedsilica photomask blank is also preferably low in molecular hydrogen,preferably less than 1×10¹⁷ molecules/cm³, and more preferably less than5×10¹⁶ molecules/cm³ of molecular hydrogen.

Reducing the OH content to less than 50 ppm, preferably less than 10ppm, and most preferably less than 1 ppm and doping the silica glasswith fluorine provides for increased transmission at 157 nm and inlowered thermal expansion. The preferred 157 nm transmissionphotolithography fluorine doped low OH fused SiO₂ glass photomasksubstrate has a transmittance of at least 80%, preferably at least about83%/cm at 157 nm, and a thermal expansion less than 0.55 ppm/° C.,preferably less than 0.53 ppm/° C.

The invention further includes a below 175 nm VUV lithography glass. Thelithography glass comprises a fused silicon oxyfluoride glass. Thesilicon oxyfluoride glass has an OH content less than 5 ppm by weight, aCl content less than 5 ppm by weight, a H₂ content less than 1×10¹⁷molecules/cm³, and a fluorine content of at least 0.1% weight %, withsaid glass having a 157 nm internal transmission of at least 80%/cm andpreferably at least 85%/cm. The silicon oxyfluoride glass has a belowfused silica coefficient of thermal expansion that is less than 0.55ppm/° C. in the room temperature to 300° C. range. Preferably thelithography glass has an internal transmission in the wavelength rangeof 157 nm to 175 nm of at least 80%/cm, and more preferably at least85%/cm. Preferably the silicon oxyfluoride lithography glass has anincrease of absorption at 215 nm of less than 0.1 optical density (log₁₀transmission) per mm when exposed to at least 0.96×10⁶ pulses of 157 nmwavelength containing F₂ excimer laser radiation at 4 m J/cm²-pulse, andmore preferably the increase of absorption at 215 nm is less than 0.05optical density, and most preferably substantially no 215 nm absorptionband is formed. Preferably the Cl content of the glass is less than 1ppm and the OH content is less than 1 ppm, and more preferably the glassconsists essentially of Si, O, and F. Preferably the glass isessentially free of metal to metal Si—Si bonds, and the glass is free ofa 165 nm absorbing center with an internal transmission at 165 run of atleast 85%/cm.

In a preferred embodiment the lithography glass is used to make a VUVtransmitting photomask where VUV light is transmitted through thephotomask, preferably with a surface of the lithography glass having apatterned deposited film (such as Cr) that forms a transmittingphotolithography mask pattern. In a further embodiment the lithographyglass is used to make a VUV phase shifting photomask where the phase ofVUV lithography light traveling through the glass is shifted andmanipulated to form constructive and/or destructive interferencepatterns. In a further embodiment the lithography glass with a loweredthermal expansion and a thermal expansion coefficient less than 0.55ppm/° C. is used to make a reflective photomask wherein a reflectivepatterned lithography mask pattern is supported by said siliconoxyfluoride glass.

The invention further includes a pattern printing method of VUVlithography which includes providing a below 164 nm radiation source forproducing VUV lithography photons, providing a silicon oxyfluoridelithography glass having less than 5 ppm by weight OH, less than 5 ppmby weight Cl, and 157 nm and 165 nm measured transmission of at least75%/5 mm, transmitting the VUV lithography photons through the providedsilicon oxyfluoride lithography glass, forming a lithography patternwith the photons, and reducing the formed lithography pattern andprojecting the formed pattern onto a VUV radiation sensitive lithographyprinting medium to form a printed lithography pattern. Providing theoxyfluoride lithography glass preferably includes lowering the VUVcutoff wavelength of the glass by providing an Si-glass formingprecursor, and lowering the H₂, the OH, and the Cl content of the directdeposit vitrified glass made therefrom and increasing the F content ofthe direct deposit vitrified glass to provide a direct deposit vitrifiedsilicon oxyfluoride glass with a 50% transmission VUV cutoff wavelengthbelow 160 nm. Preferably the provided glass consists essentially of Si,O, and F and is essentially free of Si—Si bonds.

The invention provides a 157 nm photolithography photomask substratephotomask stage and a 157 nm photolithography device (157 nmillumination system, photomask-mask stage, 157 nm projection opticssystem, 157 nm wafer stage) with such a fluorine doped low OH siliconoxyfluoride photomask silica glass substrate that has an OH content lessthan 1 ppm, a fluorine content in the range from 0.1 to 1.5 wt. %, 157nm internal transmittances of at least 50%, and preferably at least 65%,and most preferably at least 83%/cm at 157 nm, and preferably a thermalexpansion less than 0.55 ppm, preferably less than 0.53 ppm, and mostpreferably less than or equal to 0.52 ppm/° C.

Polished substrates, 25 mm×25 mm×1.5 mm thick, were prepared from thesilicon oxyfluoride glass. For comparison, 25 mm×25 mm×6.35 mm thick,substrates were also cut from a standard commercial silica photomasksubstrate. The substrates were cleaned in a sulfuric acid/peroxidesolution and mask detergent, then spin-dried and baked at 120° C. Crfilms, 100 nm thick, were deposited by sputtering. Film adhesionmeasurements were made by indentation and scratch testing using aNanoindenter II. Under the same test conditions, delamination of the Crfilm could not be induced on either type of substrate. These resultsindicate good film adhesion.

The silica glass structure can be described as a network of SiO₄tetrahedra bonded together at all four corners and randomly orientedwith respect to one another. Water is incorporated into the structure as≡Si—OH (where ≡ indicates bonding to the SiO₄ network) such that thebond to the neighboring tetrahedron is broken at the OH group. OHproduces absorption in the deep UV at <175-nm. Fluorine is similarlyincorporated into the structure as ≡Si—F with the connectivity of thenetwork being broken at the F atom. Electronic transitions associatedwith the Si—F bond are expected to be at higher energies (shorterwavelengths) than those from the Si—O network bonds.

The fluorine-doped silica structure is particularly resistant to damageby F₂ excimer laser irradiation. Here, we have shown that formation ofE′ color centers is highly suppressed in the fluorinated structure, evenin silica containing a very low concentration of fluorine. It ispossible that the fluorine reduces the number of precursor sites forcolor center formation such as weak or strained bonds andoxygen-deficient Si—Si defects.

In a preferred embodiment the invention includes a below 193 nm VUVtransmitting glass photomask substrate for photolithography atwavelengths of about 157 nm. The VUV transmitting glass substratecomprises high purity oxyfluoride glass with an OH content below 50 ppmby weight, hydrogen content below 1×10¹⁷ molecules cm³, and a fluorinecontent in the range of 0.1 to 0.4 weight percent. Preferably the glasshas a Cl content below 5 ppm, more preferably below 1 ppm, and mostpreferably with the glass being essentially free of chlorine. Preferablythe glass has a molecular hydrogen content below 3×10¹⁶ molecules/cm³,and more preferably has no detectable molecular hydrogen content.Preferably the glass has an OH content below 10 ppm by weight, morepreferably below 1 ppm by weight, and most preferably has no detectableOH content and is essentially OH free. Preferably the glass is comprisedof Si, O, and F and is essentially free of OH, Cl and H₂. Preferably thesilicon oxyfluoride glass photomask substrate has an internaltransmittance at 157 nm of at least 89%/cm, most preferably with thesubstrate having a measured transmittance of at least 79% through athickness of the photomask substrate with the substrate being about 6 mmthick, such as 6.35 mm thick.

In a preferred embodiment the invention includes a process of making aVUV transmitting glass having high resistance to optical damage toexcimer laser radiation in the 157 nm wavelength region. The processincludes forming a dry, non-porous monolithic body of transparent fusedsilicon oxyfluoride glass with a fluorine content less than 0.5 weightpercent.

The invention includes a F₂ laser-induced absorption resistant siliconoxyfluoride glass suitable for use in the 157 nm wavelength region, theglass having a stable and high transmission at 157.6 nm with a fluorinecontent less than 0.5 weight % such that the glass has a transmissionloss at 157.6 nm<1% after exposure to a F₂ excimer laser for 60 millionpulses at 0.1 mJ/cm²-pulse. The silicon oxyfluoride glass preferably isessentially free of OH groups, has less than 5×10¹⁶ molecules/cm³ ofmolecular hydrogen, and a fluorine content in the range of about 0.1 to0.4 weight %.

The invention includes a F₂ laser-induced absorption resistantlithography glass comprising a silicon oxyfluoride glass having an OHcontent less than 5 ppm, by weight, a Cl content less than 5 ppm byweight, and a fluorine content of 0.1 to 0.4 weight % with a 157 nminternal transmission of at least 80%/cm, more preferably 85%/cm.Preferably the glass has a H2 content less than 1×10¹⁷ molecules/cm³.The silicon oxyfluoride glass is resistant to laser-induced absorptionand has a 157 nm transmission loss<1% after exposure to a 157 nm laserfor 60 million pulses at 0.1 mJ/cm²-pulse. The silicon oxyfluoride glasshas a resistance to 157.6 nm induced absorption, with the fluorinecontent inhibiting 165 nm absorption oxygen-deficient centers.Preferably the glass has a 165 nm absorption less than 0.4 (absorptionunits/5 mm) after exposure to a 157 nm laser for 41.5 million pulses at2 mJ/cm²-pulse of 157 nm laser, and most preferably the 165 nmabsorption is less than 0.2 (absorption units/5 mm). Preferably the Clcontent is less than 1 ppm and the OH content is less than 1 ppm,preferably with the glass consisting essentially of Si, O and F. In anembodiment the glass is a VUV transmitting photomask. In an embodimentthe glass is a VUV phase shifting photomask. Preferably the glassphotomasks have a resistance to laser induced oxygen-deficient centers,preferably with the glass essentially free of metal to metal Si—Si bondsand free of a 165 nm absorbing center and has an internal transmissionat 165 nm of at least 85%/cm.

The invention includes a VUV pattern printing method. The patternprinting method includes providing a below 164 nm radiation source forproducing VUV photons, providing a silicon oxyfluoride glass having lessthan 5 ppm by weight OH, less than 5 ppm by weight Cl, a less than 0.5weight percent fluorine content, and a 157 nm and 165 nm measuredtransmission of at least 75%/5 mm. The method includes transmitting theVUV photons through the silicon oxyfluoride glass, forming a patternwith the VUV photons, and projecting the pattern onto a VUV radiationsensitive printing medium to form a printed pattern. In a preferredembodiment the VUV lithography pattern printing method which includesproviding a VUV lithography photon radiation source, providing a siliconoxyfluoride lithography glass with less than 1 ppm OH, transmitting theVUV lithography photons through the silicon oxyfluoride lithographyglass, forming a lithography pattern with the VUV photons, andprojecting the lithography pattern onto a VUV radiation sensitivelithography printing medium to form a printed lithography pattern.

The VUV pattern printing method preferably includes lowering the VUV cutoff wavelength of the silicon oxyfluoride glass by providing an Si-glassforming precursor and doping with an F content to provide a siliconoxyfluoride glass with a 50% transmission VUV cut off wavelength below160 nm and a 165 nm absorption less than 0.4 (absorption units/5 mm)after exposure to a 157 nm laser for 4.5 million pulses at 2mJ/cm²-pulse.

The invention provides superior transmission in the VUV with the siliconoxyfluoride glasses having high purity and being dry (<1 ppm OH). Theinventive glasses have provided 157 nm transmissions such as 79.8%/6.35mm (and 90%/cm internal T). The silicon oxyfluoride glass is preferablyutilized as components for 157 nm lithography, particularly photomasksubstrates, pellicles, thin lenses and windows. For these lithographyapplications, it is not only important that the glass exhibits a highinitial transmission, but the transmission must not decrease underexposure to the F₂ excimer laser. The glass provides a<1% transmissionloss at 157.6 nm after exposure to the F₂ excimer laser for 60 millionpulses at 0.1 mJ/cm²-pulse. Our preferred silicon oxyfluoride glasscompositions exhibit improved resistance to F₂ laser-induced absorption.

We have discovered that although the addition of fluorine to a drysilica glass improves its transmission in the vacuum ultraviolet,particularly near the UV edge (e.g., at 157 nm), high concentrations offluorine in the glass are detrimental to its laser damage resistance.Specifically, we have found that under F₂ excimer laser exposure, theinduced absorption in the glass is proportional to the fluorine contentof the glass. As the fluorine content increases, the 157.6 nm inducedabsorption increases.

Preferably our silicon oxyfluoride glass has a fluoride content lessthan 0.5 wt. % F. Our <0.5 wt. % F dry silicon oxyfluoride glassexhibits good laser durability and suitability for applicationsinvolving exposure to F₂ excimer laser radiation. Our preferredcomposition range for attaining both high initial transmission and lowinduced absorption is 0.1-0.4 wt. % F.

The invention includes a VUV transmitting silicon oxyfluoride glasshaving an OH content less than 5 ppm by weight, a fluorine content of atleast 0.1 weight %. Preferably the glass consists essentially of Si, Oand F with the glass having an internal transmission in the wavelengthrange of 157 nm to 175 nm of at least 85%/cm. Preferably the glass has a165 nm absorption less than 0.4 (absorption units/5 mm) after exposureto a 157 nm laser for 41.5 million pulses at 2 mJ/cm²-pulse.

EXAMPLES

In order to achieve the desired transmission at 157 nm (85%/6.35 mm(measured), the concentrations of OH, Cl, metals (e.g. Fe, Zr),intrinsic defects (e.g. peroxy linkage), and macroscopic defects (e.g.gas bubbles) must be minimized. The direct-to-glass, dry combustionprocess using CO fuel, is a preferred way to make dry, F-doped SiO₂glass for this and other applications. The dry, doped SiO₂ glass isproduced directly in a single step process. Due to the presence of H, amethane burner is effectively limited to production of relatively wetsoot which then requires the additional processing steps of drying andconsolidation to produce dry glass as a multi-step based process. Note,however, that although a CO burner makes it possible to produce dryglass directly, it is preferably used in conjunction with: (i) a H-freeSi source (e.g. SiCl₄) and, (ii) exclusion of outside sources of H (e.g.air leaks). Besides enabling the production of dry, doped SiO₂ glass,the invention provides benefits of (i) increased glass purity (CO reactswith most metals to form carbonyls), and (ii) decreased CF₄consumption/abatement (compared to F-doping SiO₂ soot in consolidation,the dry combustion process requires much less CF₄ to achieve the same Flevels).

Although developed for production of 157 nm photomask plates, theinvention provides the use of dry combustion to produce dry, dopedglasses for use as optical elements. The glass is preferably produceddirectly in a single step. The dopant may be F, or one or more metaloxides (including TiO₂, GeO₂, B₂O₃, P₂O₅) required to produce thedesired property.

The invention focused on the production of dry, F-doped SiO₂ glass witha measured transmission of ≧85%/6.35 mm at 157 nm for use as photomaskplates for photolithography. In order to achieve the desiredtransmission, the concentrations of the following must be minimized: (i)impurities (notably water (OH), Cl, and metals (e.g. Fe, Zr)), (ii)intrinsic defects (notably oxygen deficient centers and peroxylinkages), and (iii) macroscopic defects (notably gas bubbles).

Both experimental and theoretical work was undertaken to determine howprocess conditions affect the concentrations of impurities and defects.The main focus was on thermodynamic predictions and experimental resultsfor OH, Cl, tramp metals, and intrinsic defects. Table 1 summarizes theexperimental results of the most pertinent dry CO combustion runs

TABLE 1 157 nm transmission, OH, Cl, F, Fe and Zr analyses (bymicroprobe and ICP-MS) and the weight of glass produced in each run DC3DC4 DC5 DC6 DC7 DC8 DC10 transmission @ ˜20% thru 3 mm ˜20% thru 3 mm˜23 thru 3 mm ˜2% thru 3 mm ˜21% thru 3 mm ˜29% thru 3 mm ˜25% thru 3 mm157 nm OH-ppm <1 ppm <1 ppm <1 ppm <420 ppm <1 ppm <1 ppm <1 ppm Cl(avg. wt. %) 0.08% 0.03% 0.02% 0.00% 0.04% 10.04% 0.06% F (avg. wt. %)0.81% 0.80% 0.91% 0.13% none 0.28% 0.22% Fe (avg. wt.%) 0.0000066% [mp]0.000004% [mp] 0.0023% [mp] 0.000004% [icp] 0.000005% [icp] 0.000007%[icp] 0.000004% [icp] 0.000004% [icp] Zr (avg. wt. %) 0.0000458% [mp]0.000045% [mp] 0.0052% [mp] 0.000006% [icp] 0.00002% [icp] 0.00003%[icp] 0.000001% [icp] 0.00125% [icp] Si source SiCl₄ SiCl₄ SiCl₄ omctsSiCl₄ SiCl₄ SiCl₄ glass weight ˜570 g ˜250 g ˜250 g ˜850 g ˜250 g ˜200 g˜200 g Si yield 24.7% 13.0% 13.5% 34.5% 12.5% 10.6% 10% experimentalincrease O₂ decrease SiCl₄ OMCTS Si SiCl₄ Si source decrease CF₄purified CO changes vs. increase CF₄ source no CF₄ vs. DC6 Al₂O₃ brickprevious run

Water

Water (as OH in the glass) decreases transmission in the VUV. Forinstance, HPFS™ (Corning code 7980), fused silica produced by the flamehydrolysis process (where CH₄ is the fuel and OMCTS is the Si source),does not transmit light at wavelengths below ˜170 nm because of its highwater content (˜800 ppm OH typical), as shown in FIG. 15 (1+14).Minimization of water in the glass was the main driver for developmentof the dry combustion process, where H was eliminated from the burnerinputs by replacing CH₄ with CO, and OMCTS with SiCl₄, in asingle-burner furnace (FIG. 12).

Once the nitrogen curtain was deployed to help exclude moist air fromthe furnace chamber, dry glass (<1 ppm OH) was produced, with andwithout F-doping, but always using CO as the fuel and SiCl₄ as the Sisource. Although it significantly reduced the Cl-content of the glass,when OMCTS, a Cl-free Si source, was substituted for SiCl₄ in run DC6,it resulted in glass that was too wet (˜420 ppm OH) and which does nottransmit in the VUV (FIG. 15 (1+14) shows the absorption edge due towater).

Chlorine

Like water, Cl decreases transmission in the VUV, with a strongcorrelation between absorption at the UV edge and residual chlorine insilica glass, and since Cl has a relatively high absorptioncross-section at 157 nm (6.32×10⁻²⁰ cm²—about 40% of that for OH), onlya very small amount of Cl can be tolerated in the glass for thisapplication. Unfortunately, but not surprisingly, the use of SiCl₄, aH-free but Cl-containing Si source, leads to high levels of Cl in glassproduced by the dry CO combustion process. The dry combustion processproduced glass containing ˜0.08 wt % Cl (DC3) from SiCl₄. Minimizationof Cl in the glass therefore became the main focus in the development ofthe dry combustion process.

The following approaches for minimization of Cl in the glass wereevaluated: (i) use Cl-free Si source (eliminate Cl in glass byeliminating Cl from the system), (ii) increase O₂ (minimize Cl in glassby increasing O₂ in combustion atmosphere), (iii) increase CF₄ (minimizeCl in glass by increasing F incorporation).

Theory and/or experiment indicate that all four of these approaches willlead to glass with lower Cl content; however, only (ii) and (iii) couldbe implemented without introducing H into the system. A Cl content of˜0.02 wt % (this is a 75% decrease from the “initial condition,” DC3)was achieved (DC5) using a combination of increased O₂ and increasedCF₄. Si(NCO)₄ provides a beneficial feedstock.

The basic idea behind (i) is to entirely exclude Cl from the system, andis clearly the best way to minimize/eliminate Cl in the glass. The basicidea behind (ii) is that increasing O₂ in the combustion atmosphere willhinder the Cl-incorporation reaction, i.e. shift the followingequilibrium to the left:

0.5 Cl₂ (gas)+SiO₂ (glass)=SiO_(1.5)Cl (Cl-doped glass)+O₂ (gas)

FIG. 16 (14+2) shows the predicted effect of increased O₂.

The basic idea behind (iii) is that F and Cl occupy the same sites inSiO₂ and so increasing F concentration should decrease Cl concentrationin the glass. Both theoretical and experimental evidence suggest thatthe approach is useful. Combined with increased O₂ and decreased SiCl₄flow, increased F led to the lowest Cl glass produced by dry combustionwith SiCl₄ as the Si source (DC5, ˜0.02 wt % Cl).

Tramp Metals

Since tramp metals are known to reduce transmission in the UV, theobjective was to produce high-purity glass with a minimum of trampmetals. However, given the relatively high Cl concentrations in theseglasses, the transmission loss due to the presence of tramp metals is asecond order effect. As shown in Table 1, the concentrations of Fe andZr (the main tramp metals) were significantly decreased by the use ofpurified CO and by substituting Al₂O₃ for zircon refractories in DC10(Fe was reduced by a factor of ˜2, and Zr was reduced by a factor of˜10).

The absorption effect of contaminant species were studied in the siliconoxyflouride glass with the results as below.

TABLE 2 Absorption cross-sections at 157 nm and 193 nm For 1%/cm For1%/cm T loss T loss σ × 10⁻²⁰ cm² σ × 10⁻²⁰ cm² at 157 nm at 193 nmSpecies (157 nm) (193 nm) (ppm wt.) (ppm wt.) *Cl 6.3 2 (Cl) *OH 16.80.4 (OH) Al 33.6 0.56 (Al₂O₃) Na 57.9 1.6 0.2 (Na₂O) 7 (Na₂O) Zr 748 6.40.06 (ZrO₂) 7 (ZrO₂) Fe 897 658 0.03 (FeO) 0.04 (FeO)

This Absorption cross-sections at 157 nm and 193 nm study showed thebenefit of utilizing alumina refractory in the making of the 157 nmglass.

Intrinsic Defects

Both oxidized and reduced defects are known to decrease transmission at157 nm, so the objective is to produce neutral (i.e. neither oxidizednor reduced) glass with a minimum of both types of defects. It isthought that the Si metal—metal bond defect is responsible for anabsorption band at 165 nm—an absorption which significantly diminishestransmission at 157 nm. Intrinsic defects naturally occur in silicateglasses prepared at high temperatures under equilibrium conditions, andboth temperature and stoichiometry are known to influence theconcentrations. Experimentally we have shown that the 165 nm absorptionband can be minimized/eliminated by decreasing the concentration of F inthe glass. As shown in FIG. 1, the 165 nm absorption band appeared inall cases except DC7 (no F), DC8 (low F—0.28 wt %), and DC10 (low F—0.22wt %).

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A below 193 nm VUV transmitting glass photomasksubstrate for photolithography at wavelengths of about 157 nm, saidglass photomask substrate comprising a dry high purity direct depositvitrified silicon oxyfluoride glass with an OH content below 20 ppm byweight, a Cl content below 0.1% by weight, and a fluorine content in therange of 0.01 to 7 weight percent.
 2. A glass photomask substrate asclaimed in claim 1, wherein said dry high purity direct depositvitrified silicon oxyfluoride glass has a fluorine content in the rangeof 0.01 to 2 weight percent.
 3. A glass photomask substrate as claimedin claim 1, wherein said dry high purity direct deposit vitrifiedsilicon oxyfluoride glass has a fluorine content in the range of 0.01 to0.5 weight percent.
 4. A glass photomask substrate as claimed in claim1, wherein said dry high purity direct deposit vitrified siliconoxyfluoride glass has a fluorine content in the range 0.1 to 0.4 weightpercent.
 5. A glass photomask substrate as claimed in claim 1, whereinsaid dry high purity direct deposit vitrified silicon oxyfluoride glasshas a Cl content≦0.08% by weight.
 6. A glass photomask substrate asclaimed in claim 1, wherein said dry high purity direct depositvitrified silicon oxyfluoride glass has a Cl content≦0.06% by weight. 7.A glass photomask substrate as claimed in claim 1, wherein said dry highpurity direct deposit vitrified silicon oxyfluoride glass has a Clcontent≦0.04% by weight.
 8. A glass photomask substrate as claimed inclaim 1, wherein said dry high purity direct deposit vitrified siliconoxyfluoride glass has a Cl content≦0.03% by weight.
 9. A glass photomasksubstrate as claimed in claim 1, wherein said dry high purity directdeposit vitrified silicon oxyfluoride glass has a Cl content≦0.02% byweight.
 10. A glass photomask substrate as claimed in claim 1, whereinsaid dry high purity direct deposit vitrified silicon oxyfluoride glasshas a Cl content below 5 ppm.
 11. A glass photomask substrate as claimedin claim 1, wherein said dry high purity direct deposit vitrifiedsilicon oxyfluoride glass has a Cl content below 1 ppm.
 12. A glassphotomask substrate as claimed in claim 1, wherein said dry high puritydirect deposit vitrified silicon oxyfluoride glass is Cl-free.
 13. Aglass photomask substrate as claimed in claim 1, wherein said dry highpurity direct deposit vitrified silicon oxyfluoride glass has an OHcontent below 10 ppm by weight.
 14. A glass photomask substrate asclaimed in claim 1, wherein said dry high purity direct depositvitrified silicon oxyfluoride glass has an OH content below 1 ppm byweight.
 15. A glass photomask substrate as claimed in claim 1, whereinsaid dry high purity direct deposit vitrified silicon oxyfluoride glasshas a hydrogen content below 1×10¹⁷ molecules/cm³.
 16. A glass photomasksubstrate as claimed in claim 1, wherein said dry high purity directdeposit vitrified silicon oxyfluoride glass has a Fe content no greaterthan 0.00004% by weight.
 17. A glass photomask substrate as claimed inclaim 1, wherein said dry high purity direct deposit vitrified siliconoxyfluoride glass has a Zr content no greater than 0.00004% by weight.18. A glass photomask substrate as claimed in claim 1, wherein said dryhigh purity direct deposit vitrified silicon oxyfluoride glass containsa plurality of O₂ molecules.
 19. A glass photomask substrate as claimedin claim 1, wherein said dry high purity direct deposit vitrifiedsilicon oxyfluoride glass contains a plurality of O₂ molecules with anO₂ concentration of at least 10¹⁵ O₂ mole/cc.